1. Field of the Invention
The present invention relates to a motor control method and a motor control apparatus for a dental handpiece that cuts teeth or forms a root canal with a cutting tool.
2. Description of the Related Art
To cut teeth or form a root canal, a dental handpiece provided with an electrical motor as a driving source is used. The operator selects one suitable for the operation from among a plurality of types of cutting tools.
A known method of controlling the motor of such a dental handpiece is a feedback control, in which the motor is controlled so as to keep the preset rotational speed by detecting the rotational speed of the motor and increasing or decreasing the motor driving voltage based on the detected rotational speed.
If the load torque on the cutting tool increases and the rotational speed decreases, a motor control apparatus based on the feedback control increases the motor driving voltage, thereby restoring the preset rotational speed.
However, when the feedback control is used for root canal formation or other operations that involve using a thin cutting tool that is easily broken, there arises an operational problem that an excessive torque can be applied to the cutting tool, and the cutting tool can be broken or jammed into a root canal.
To solve such a problem, the motor control apparatus proposed in Japanese Patent No. 3264607 does not perform the feedback control but controls the motor output characteristics so as to decrease the rotational speed as the load torque applied to the cutting tool increases. When the load torque on the cutting tool reaches a preset reference torque, the motor control apparatus performs one of three operations: (1) to decrease the rotational speed of the motor, (2) to stop the motor, and (3) to reverse the rotation of the motor.
In this way, the motor control apparatus according to Japanese Patent No. 3264607 can prevent the cutting tool from being broken and can remove the cutting tool from the root canal by reversing the rotation of the motor, and thus, the operability is improved.
However, although the motor control apparatus disclosed in Japanese Patent No. 3264607 can prevent the cutting tool from being broken or jammed into a root canal, the motor control apparatus is inferior to the feedback control in cutting efficiency because the rotational speed of the cutting tool decreases as the load torque increases.
In addition, in the above-described case (2) where the motor is stopped, the rotation of the motor is stopped by stopping application of the driving voltage to the motor as shown in the diagrams (a) and (b) in FIG. 5 of Japanese Patent No. 3264607. As a result, the torque of the motor (the cutting tool) in the direction of rotation suddenly drops, thereby causing a shock (referred to as discomfort hereinafter) to the patient and the operator.
In the above-described case (3) where the rotation of the motor is reversed, the driving voltage to the motor is stopped to stop the rotation of the motor before the reversal, so that the discomfort occurs as in the case (2) described above.
The present invention has been accomplished in view of such problems, and an object of the present invention is to provide a motor control method and a motor control apparatus for a dental handpiece that can prevent a cutting tool from being broken, allow efficient cutting and reduce a discomfort to a patient and an operator during operation.